Fungicidal control of metalaxyl-insensitive strains of Bremia lactucae on lettuce

Strains of B. lactucae insensitive to 0.01 g a.i. l⁻¹ metalaxyl have been detected in all the lettuce-growing districts of South Australia. Despite this, applications of a formulated mixture of 0.25 g a.i. l⁻¹ metalaxyl and 2 g a.i. l⁻¹ mancozeb every 7–10 days controlled downy mildew in field experiments, where similar applications of either metalaxyl or mancozeb alone were ineffective. Protectant schedules of either 0.3 g a.i. l⁻¹ dimethomorph or 2–2.4 g a.i. l⁻¹ phosphonic acid also controlled downy mildew, but were less effective than the metalaxyl/mancozeb formulation. Mixtures of either dimethomorph or phosphonic acid with mancozeb enabled the time between the sprays to be increased to 14 days without significantly reducing control. Fungicide applications increased marketable yield by 60% compared with that of the unsprayed plants. Drenching seedlings with 4 g a.i. l⁻¹ phosphonic acid controlled downy mildew for at least 14 days and should prevent the spread of the disease from plant production areas.